A good solution to tri-state checkbox

I'm working on a web system, that will have an admin area and a normal user area. There are two types of normal users: Type A and Type B.

In the admin area, the administrator will register categories. Some categories will appear only to the users of Type A, some others only to the Type's B users, and the another to both.

I would like a suggestion to a good control to the administrator input this value, I thought in two checkboxes (with on/off style):

• Show for A users
• Show for B Users

The problem with this solution is to validate when the admin leave the two checkboxes empty. I want to a solution that the admin can't input the value in a wrong way. So I thought in use radio buttons instead, like this:

• Show only for A users
• Show only for B users
• Show to both

But with this, I fell myself afraid to show so much options to the admin user. This screen will be used so many times to register a large number of categories and I don't want get my users tired of.

What would be a better solution?